The sole difference in the status of the firstborn son as compared with that of his brothers is his right to a greater share in their father's inheritance. This status is known as bekhor le-naḥalah (firstborn or primogeniture as to inheritance) and derives from the verse "he must acknowledge the firstborn the son of the unloved one, and allot to him a double portion of all he possesses; since he is the first fruit of his vigor, the birthright is his due" (Deut. 21:15–17). The firstborn in this context is the first son born to the father, even if not so to the mother, since it is written, "the first fruits of his vigor" (Bek. 8:1 and see commentators). Even if such a son is born of a prohibited union, e.g., the son of a priest and a divorced woman, or a mamzer born as first son to his father – he is included, on the strength of the words "he must acknowledge the firstborn, the son of the unloved one" (Deut., loc. cit.), the term a "loved" or an "unloved" wife being interpreted as relating only to the question whether the wife's marriage was "loved" or "unloved," i.e., permitted or prohibited (Yev. 23a and see Rashi and Posekim ad loc.). The prerogative of the firstborn never extends to a daughter, not even in a case where she has a right of inheritance (Sif. Deut. 215; see *Inheritance). A son born to a proselyte to Judaism, who had sons before he became a proselyte, does not enjoy the prerogative of a bekhor le-naḥalah, since he is not "the first fruits of his vigor" (Yev. 62a; Bek. 47a; Posekim ad loc.); on the other hand, if an Israelite had a son by a non-Jewish woman and thereafter has a son by a Jewish woman, the latter son does enjoy the prerogative, since the former is called her, and not his, son (Maim. Yad, Naḥalot 2:12). A first son who is born after his father's death, viz., if the mother gives birth to twins, is not considered a bekhor le-naḥalah since it is written "he must acknowledge" (Deut. 21:17) and the father is no longer alive to do so (BB 142b; Rashbam and Posekim ad loc.).